The present invention relates to a gas turbine engine used for aircraft, and more particularly to a method and a system for gas turbine engine starting.
Gas turbine engine starting typically requires the use of a powerful electric or pneumatic starter, which is usually mounted in an accessory gear box, to drive the compressor/turbine rotor, accessories connected to the accessory gear box, such as generators, pumps, air conditioning, etc., and engine system components integrated into the gear box, during the start sequence of the gas turbine engine. The starting cycle is completed when the gas turbine engine reaches a self-sustaining operative condition. In conventional practice, the electric or pneumatic starters provide drive torque during the start cycle to overcome the drag from the compressor and turbine rotors, and loads resulting from all of the accessories connected to the accessory gearbox drive train driven by the starter.
The power source for actuating a starter motor of a gas turbine engine used in aircraft generally includes electric power stored in batteries or pressurized fluid provided by an APU (Auxiliary Power Unit). The energy provided by those power sources is limited and therefore it is desirable to reduce the torque requirement for gas turbine engine starting.
Clutches are well known in the art and are used in gear trains of gas turbine engines for selectively establishing or terminating torque transmission through the gear trains. U.S. Pat. No. 5,201,798, issued to Hogan on Apr. 12, 1993 describes one example of the use of one way clutches in the gear train of a gas turbine engine. A multiple function integrated power unit for use aboard aircraft includes two turbine engines operable under different conditions, an integrating gear box receiving power from the two engines selectively, and a plurality of accessory devices receiving shaft power from either of the two engines through the gear box. One clutch is used to connect a starter to the gear train to drive the first engine. After attaining self-sustaining speed, the first engine accelerates under its own power to its operating speed, and the flow of pressurized air to the starter is discontinued. The clutch ensures that the starter is not driven by the first engine. The first engine provides shaft power to the gear train within the gear box through a sprag clutch. The multiple integrated power unit also includes a second engine which is connected to the gear train within the gear box through another sprag clutch so that when the accessories are driven by either one of the two engines the other engine will not be driven.
U.S. Pat. No. 4,257,281, issued to Bunger on Mar. 24, 1981 also describes the use of an over-running clutch incorporated into an engine starter and accessory drive system, so that the clutch is engaged to transfer torque from the starter to the engine and the accessory devices during the engine starting, and the clutch is disengaged to prevent torque from being transferred to the starter during engine operation.
Nevertheless, those systems generally prevent torque from being transmitted to the starter or other rotator during engine operation, and does not improve engine starting. Therefore, there is a need for a method and a system for improved engine starting.
One object of the present invention is to provide a method for gas turbine engine starting with a reduced load.
Another object of the present invention is to provide a gas turbine engine starting system which can start the gas turbine engine with a reduced load.
In accordance with one aspect of the present invention, the method of gas turbine engine starting comprises using a starter connected to a rotor of a gas turbine engine to start the gas turbine engine while terminating or reducing torque transmission from the starter to accessory devices until a gas turbine engine starting cycle is completed and the gas turbine engine is operating under a self-sustaining condition.
A clutch is preferably used for completely or partially disconnecting a drive system of the accessory devices from the rotor of the gas turbine engine before the gas turbine engine starting cycle begins. The drive system is used for distribution of torque from the rotor of the gas turbine engine to the accessory devices during gas turbine engine operation. Partial re-engagement of accessory drives can be accomplished to provide necessary drive to fuel pumps and oil pumps during the start sequence as required. When the gas turbine engine starting cycle is completed and the gas turbine engine is operating in the self-sustaining condition, the drive system of the accessory devices is fully reconnected to the rotor of the gas turbine engine. In one embodiment of the present invention, the clutch is a magnetic fluid clutch and it is controlled by an electronic controller of the gas turbine engine, or the air craft.
In accordance with another aspect of the present invention, a system is provided for starting a gas turbine engine. The system comprises a starter coupled to a rotor of the gas turbine engine by means of a torque transmitting mechanism. A clutch is provided to couple a drive system of accessory devices with the torque transmitting mechanism for controlling torque transmission from the torque transmitting mechanism to the drive system of the accessory devices while maintaining the torque transmission from the starter to the rotor of the gas turbine engine. Thus, torque required for the gas turbine engine starting can be reduced by completely or partially declutching the clutch during a gas turbine engine starting cycle.
The clutch is preferably a magnetic fluid clutch and is controlled by an electronic controller which is incorporated into the engine control system or the aircraft control system. The drive system of the accessory devices preferably comprises an accessory gear box for distribution of torque from the rotor of the gas turbine engine to the accessory devices during gas turbine engine operation. The magnetic fluid clutch according to one embodiment of the present invention includes a rotating shaft having a first end and a second end. A casing rotatably surrounds the rotating shaft and contains a magnetic fluid therein. The first end of the rotating shaft is coupled to the starter and the second end of the rotating shaft is connected to the rotor of the gas turbine engine. The casing is coupled to a gear train of the accessory gear box.
The magnetic fluid clutch according to another embodiment of the present invention, includes a stationary casing containing the magnetic fluid. A first rotating shaft and a second rotating shaft are provided. The respective rotating shafts extend oppositely and outwardly from the inside of the casing, and are rotatable relative to each other and relative to the casing. Each rotating shaft has a plate affixed thereto. The two plates are axially spaced apart and are disposed in the magnetic fluid contained within the casing. An electric coil is provided for applying an electric-magnetic field to the magnetic fluid. The electric coil is controlled by an electronic controller which could be incorporated into the control system of the gas turbine engine or the control system of the aircraft. The first shaft of the magnetic fluid clutch is coupled to the starter and is also connected to the rotor of the gas turbine engine. The second rotor shaft of the magnetic fluid clutch is connected to a gear train of the accessory gear box.
In the system of the present invention, torque transmission between the starter and the rotor of the gas turbine engines is ensured by the structure, and torque transmission from the engine rotor/starter system to the accessory devices is controlled by the magnetic fluid clutch. Thus, the accessory devices can be completely or partially disconnected from the engine rotor/starter system during a starting cycle to reduce a total amount of torque provided by the starter for engine starting.
Reduced load from accessories, is especially beneficial during starting at cold temperatures. Additionally, the clutch can disengage accessories during engine shut down, thereby avoiding injection of excess fuel and oil into the engine.
Other advantages and features of the present invention will be better understood with reference to preferred embodiments of the present invention described hereinafter.